SFINAEd fixed_variant relational operators (#33)

* updated CI
* SFINAEd fixed_variant's relational operators
* updated docs
* stylistic
* updated release notes
* reformulated to try to make VS 2015/2017 pass
* reverted, tried dropping convertible_to_bool
* reverted, used CWG1558-resistent local impl of mp_valid
* added missing ::type
* used battle-tested Boost type traits
* stylistic

.github/workflows/ci.yml

@@ -11,6 +11,7 @@ on:
 
 env:
   UBSAN_OPTIONS: print_stacktrace=1
+  ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
 
 jobs:
   posix:
@@ -175,12 +176,15 @@ jobs:
             container: ubuntu:24.04
             os: ubuntu-latest
             install: clang-18
-          - toolset: clang
-            cxxstd: "11,14,17,20,2b"
-            os: macos-13
           - toolset: clang
             cxxstd: "11,14,17,20,2b"
             os: macos-14
+          - toolset: clang
+            cxxstd: "11,14,17,20,23,2c"
+            os: macos-15
+          - toolset: clang
+            cxxstd: "11,14,17,20,23,2c"
+            os: macos-26
 
     runs-on: ${{matrix.os}}
     container:
@@ -295,3 +299,4 @@ jobs:
         run: |
           cd ../boost-root
           b2 -j3 libs/%LIBRARY%/test toolset=${{matrix.toolset}} cxxstd=${{matrix.cxxstd}} address-model=${{matrix.addrmd}} variant=debug,release embed-manifest-via=linker
+

doc/poly_collection.qbk

@@ -1,7 +1,7 @@
 [library Boost.PolyCollection
   [quickbook 1.6]
   [authors [López Muñoz, Joaquín M]]
-  [copyright 2016-2025 Joaquín M López Muñoz]
+  [copyright 2016-2026 Joaquín M López Muñoz]
   [category containers]
   [id poly_collection]
   [dirname poly_collection]
@@ -1204,6 +1204,15 @@ to allow for the definition of container-level `operator<` and related operators
 
 [section Release notes]
 
+[section Boost 1.92]
+
+* Made [' `variant_collection_value_type_impl`]`<Ts...>`'s relational operators available
+only when the corresponding relational operator is available for all `Ts...`, in
+accordance with WG21 paper
+[@https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2944r3.html P2944R3].
+
+[endsect]
+
 [section Boost 1.88]
 
 * Added `boost::variant_collection`, a closed polymorphic collection

doc/reference.qbk

@@ -1416,6 +1416,8 @@ a pointer to the contained value otherwise.
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& x,`[br]
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& y);`
 
+[*Constraints:]  `std::declval<const Ti&>()==std::declval<const Ti&>()` is a valid expression
+convertible to `bool` for all `Ti` in `Ts...`.[br]
 [*Returns:] `x.index()==y.index()&&get<I>(x)==get<I>(y)` with `I==x.index()`.
 
 [#poly_collection.reference.header_boost_poly_collection_va0.alias_template_variant_collectio.operator_ne]
@@ -1425,6 +1427,8 @@ a pointer to the contained value otherwise.
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& x,`[br]
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& y);`
 
+[*Constraints:]  `std::declval<const Ti&>()!=std::declval<const Ti&>()` is a valid expression
+convertible to `bool` for all `Ti` in `Ts...`.[br]
 [*Returns:] `x.index()!=y.index()||get<I>(x)!=get<I>(y)` with `I==x.index()`.
 
 [#poly_collection.reference.header_boost_poly_collection_va0.alias_template_variant_collectio.operator_lt]
@@ -1434,6 +1438,8 @@ a pointer to the contained value otherwise.
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& x,`[br]
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& y);`
 
+[*Constraints:]  `std::declval<const Ti&>()<std::declval<const Ti&>()` is a valid expression
+convertible to `bool` for all `Ti` in `Ts...`.[br]
 [*Returns:] `x.index()<y.index()||x.index()==y.index()&&get<I>(x)<get<I>(y)` with `I==x.index()`.
 
 [#poly_collection.reference.header_boost_poly_collection_va0.alias_template_variant_collectio.operator_gt]
@@ -1443,6 +1449,8 @@ a pointer to the contained value otherwise.
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& x,`[br]
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& y);`
 
+[*Constraints:]  `std::declval<const Ti&>()>std::declval<const Ti&>()` is a valid expression
+convertible to `bool` for all `Ti` in `Ts...`.[br]
 [*Returns:] `x.index()>y.index()||x.index()==y.index()&&get<I>(x)>get<I>(y)` with `I==x.index()`.
 
 [#poly_collection.reference.header_boost_poly_collection_va0.alias_template_variant_collectio.operator_le]
@@ -1452,6 +1460,8 @@ a pointer to the contained value otherwise.
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& x,`[br]
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& y);`
 
+[*Constraints:]  `std::declval<const Ti&>()<=std::declval<const Ti&>()` is a valid expression
+convertible to `bool` for all `Ti` in `Ts...`.[br]
 [*Returns:] `x.index()<y.index()||x.index()==y.index()&&get<I>(x)<=get<I>(y)` with `I==x.index()`.
 
 [#poly_collection.reference.header_boost_poly_collection_va0.alias_template_variant_collectio.operator_ge]
@@ -1461,6 +1471,8 @@ a pointer to the contained value otherwise.
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& x,`[br]
 [^ \u00a0\u00a0]`const `[^ /variant_collection_value_type_impl/]`<Ts...>& y);`
 
+[*Constraints:]  `std::declval<const Ti&>()>=std::declval<const Ti&>()` is a valid expression
+convertible to `bool` for all `Ti` in `Ts...`.[br]
 [*Returns:] `x.index()>y.index()||x.index()==y.index()&&get<I>(x)>=get<I>(y)` with `I==x.index()`.
 
 [#poly_collection.reference.header_boost_poly_collection_va0.alias_template_variant_collectio.visit]

include/boost/poly_collection/detail/fixed_variant.hpp

@@ -1,4 +1,4 @@
-/* Copyright 2024-2025 Joaquin M Lopez Munoz.
+/* Copyright 2024-2026 Joaquin M Lopez Munoz.
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -24,6 +24,12 @@
 #include <boost/mp11/utility.hpp>
 #include <boost/poly_collection/detail/is_equality_comparable.hpp>
 #include <boost/poly_collection/detail/is_nothrow_eq_comparable.hpp>
+#include <boost/type_traits/has_equal_to.hpp>
+#include <boost/type_traits/has_greater.hpp>
+#include <boost/type_traits/has_greater_equal.hpp>
+#include <boost/type_traits/has_less.hpp>
+#include <boost/type_traits/has_less_equal.hpp>
+#include <boost/type_traits/has_not_equal_to.hpp>
 #include <boost/type_traits/is_constructible.hpp>
 #include <cstddef>
 #include <limits>
@@ -511,7 +517,14 @@ struct eq_
   bool operator()(const T& x,const T& y)const{return x==y;}
 };
 
-template<typename... Ts>
+template<typename T>
+using eq_constraint=has_equal_to<T,T,bool>;
+
+template<
+  typename... Ts,
+  typename std::enable_if<
+    mp11::mp_all<eq_constraint<Ts>...>::value>::type* =nullptr
+>
 bool operator==(
   const fixed_variant<Ts...>& x,const fixed_variant<Ts...>& y)
 {
@@ -526,7 +539,14 @@ struct neq_
   bool operator()(const T& x,const T& y)const{return x!=y;}
 };
 
-template<typename... Ts>
+template<typename T>
+using neq_constraint=has_not_equal_to<T,T,bool>;
+
+template<
+  typename... Ts,
+  typename std::enable_if<
+    mp11::mp_all<neq_constraint<Ts>...>::value>::type* =nullptr
+>
 bool operator!=(
   const fixed_variant<Ts...>& x,const fixed_variant<Ts...>& y)
 {
@@ -542,7 +562,14 @@ struct lt_
   bool operator()(const T& x,const T& y)const{return x<y;}
 };
 
-template<typename... Ts>
+template<typename T>
+using lt_constraint=has_less<T,T,bool>;
+
+template<
+  typename... Ts,
+  typename std::enable_if<
+    mp11::mp_all<lt_constraint<Ts>...>::value>::type* =nullptr
+>
 bool operator<(
   const fixed_variant<Ts...>& x,const fixed_variant<Ts...>& y)
 {
@@ -558,7 +585,14 @@ struct lte_
   bool operator()(const T& x,const T& y)const{return x<=y;}
 };
 
-template<typename... Ts>
+template<typename T>
+using lte_constraint=has_less_equal<T,T,bool>;
+
+template<
+  typename... Ts,
+  typename std::enable_if<
+    mp11::mp_all<lte_constraint<Ts>...>::value>::type* =nullptr
+>
 bool operator<=(
   const fixed_variant<Ts...>& x,const fixed_variant<Ts...>& y)
 {
@@ -574,7 +608,14 @@ struct gt_
   bool operator()(const T& x,const T& y)const{return x>y;}
 };
 
-template<typename... Ts>
+template<typename T>
+using gt_constraint=has_greater<T,T,bool>;
+
+template<
+  typename... Ts,
+  typename std::enable_if<
+    mp11::mp_all<gt_constraint<Ts>...>::value>::type* =nullptr
+>
 bool operator>(
   const fixed_variant<Ts...>& x,const fixed_variant<Ts...>& y)
 {
@@ -590,7 +631,14 @@ struct gte_
   bool operator()(const T& x,const T& y)const{return x>=y;}
 };
 
-template<typename... Ts>
+template<typename T>
+using gte_constraint=has_greater_equal<T,T,bool>;
+
+template<
+  typename... Ts,
+  typename std::enable_if<
+    mp11::mp_all<gte_constraint<Ts>...>::value>::type* =nullptr
+>
 bool operator>=(
   const fixed_variant<Ts...>& x,const fixed_variant<Ts...>& y)
 {